The present invention relates to liquid crystal compositions which exhibit a smectic A phase of relatively low melting point and wide smectic range. Such compositions may be used in liquid crystal cells, not only in display or optical information processing applications, but also for instance in temperature sensing applications. The operation of such cells may involve laser addressing, thermal or electrical addressing.
Smectic A liquid crystals are highly viscous and therefore often require substantially greater energy, in the form of heat, light or electric field, to effect any molecular change in the phase than do typical nematic or cholesteric liquid crystals.
From a practical point of view, for most applications, it is desirable for the smectic A phase to exist at room temperature. It is also desirable for this smectic A phase to be present over a relatively wide temperature range in order that it may be used, without recourse to thermostatting, in equipment that is designed for use in situations involving a wide range of service temperature. Typical smectic A mixtures that are currently commercially available exhibit somewhat inconveniently narrow ranges of smectic A phase for a number of applications. Thus it is seen from the following table of smectic A phase materials commercially available from BDH Chemicals Ltd. under the designations S1 to S5 that the lower end of the service temperature range does not extend significantly below the freezing point of water, while the upper end is limited to between about 40.degree. and 55.degree. C.
______________________________________ K-S.sub.A S.sub.A -N N-I ______________________________________ S1 5 40 43.degree. C. S2 -1 48 49 S3 0 55 61 S4 0 54 57 S5 1 55.5 61 ______________________________________
An object of the present invention is the provision of liquid crystal mixtures with an extended temperature range for the smectic A phase, typically one that extends to cover the range from about -10.degree. C. to about +70.degree. C., but the scope of the invention is not restricted to this temperature range.
It should be noted that, to be useful in the types of smectic A displays described above, the smectic A liquid crystal mixtures must also possess other appropriate properties in addition merely to the smectic A phase. For example, many of the display principles involved depend upon reorienting the liquid crystal to a homeotropic state by means of an external electric field to erase information written on the display. This reorientation may be performed in the smectic phase, in the nematic phase, or in the isotropic phase just above the N-I transition where sufficient post-transitional order remains. So that this reorientation may be achieved by the minimal electric field (i.e. the minimum applied voltage) it is desirable that the smectic A components possess a relatively high positive dielectric anisotropy. Furthermore, many of the types of smectic displays described above depend for their optical contrast on a difference in texture between the part upon which information is written and the part upon which information is not written. Various means are adopted according to the display technology to induce a light scattering texture in the part where information is written. The efficacy of the scattering process towards incident light of visible wavelengths depends upon the optical birefringence of the smectic liquid crystal, and this, therefore, preferably should also be relatively high. Furthermore, an attractive feature of the smectic A type of display depends upon the fact that the scattering texture so induced is permanent so that information written into the display does not need to be refreshed continuously as in the twisted nematic or phase-change types of display, thus simplifying the electronic driving circuits. Now experience has shown that the stability of the scattering texture does not persist throughout the whole of the S.sub.A phase but diminishes with increasing temperature of the display because at a particular temperature the scattering texture is reoriented from the surrounding unwritten homeotropic part, thus spontaneously erasing the written information. It is important that the temperature at which reorientation occurs be well above any temperature that the display will experience. In general this temperature is proportional to the S.sub.A -N transition temperature of the mixture. Finally of course, the additives incorporated for the purpose of raising the S.sub.A -N transition temperature must be stable optically and thermally, and must not suffer electrochemical degradation under the voltages necessary to operate the display.
It is an object of this invention to provide additives with the properties described above.